Hydrogen-selective membrane

ABSTRACT

A hydrogen-selective membrane comprises a tubular porous ceramic support having a palladium metal layer deposited on an inside surface of the ceramic support. The thickness of the palladium layer is greater than about 10 μm but typically less than about 20 μm. The hydrogen permeation rate of the membrane is greater than about 1.0 moles/m 2 .s at a temperature of greater than about 500° C. and a transmembrane pressure difference of about 1,500 kPa. Moreover, the hydrogen-to-nitrogen selectivity is greater than about 600 at a temperature of greater than about 500° C. and a transmembrane pressure of about 700 kPa. Hydrogen can be separated from a mixture of gases using the membrane. The method may include the step of heating the mixture of gases to a temperature of greater than about 400° C. and less than about 1000° C. before the step of flowing the mixture of gases past the membrane. The mixture of gases may include ammonia. The ammonia typically is decomposed to provide nitrogen and hydrogen using a catalyst such as nickel. The catalyst may be placed inside the tubular ceramic support. The mixture of gases may be supplied by an industrial process such as the mixture of exhaust gases from the IGCC process.

GOVERNMENTAL SUPPORT

This invention was made in part with governmental support provided bythe United States Department of Energy under contract numberDE-AC21-89MC26373. The United States government may have certain rightsin this invention.

FIELD OF THE INVENTION

This invention concerns a palladium-ceramic membrane and a method forselectively separating hydrogen from a mixture of gases using themembrane, particularly at elevated temperatures.

BACKGROUND OF THE INVENTION

A number of industrial processes require selectively removing hydrogenfrom a reaction media. A first example of such a process is theformation of hydrogen gas by the decomposition of ammonia. Ammoniadecomposes to form nitrogen and hydrogen. Removing either hydrogen ornitrogen from the system favors the decomposition of ammonia.Selectively removing hydrogen from the mixture depends on severalfactors, including the temperature and pressure of the reaction mixture.At high temperatures and pressures, the task of separating a particulargas from a mixture of gases requires considerable effort. Furthermore,conventional packed-bed reactors cannot decompose ammonia efficiently.This is primarily because the high concentrations of hydrogen andnitrogen gas involved in the process favor the formation of ammonia.

A second example of an industrial process that may be aided by theselective removal of hydrogen from a mixture of gases is the integrated,gasification combined cycle process (IGCC). The goal of this process isto produce a synthetic gas that is used to power a gas turbinegenerator. The IGCC process produces a synthetic gas stream thatcontains trace amounts of ammonia and other impurities, such as hydrogensulfide (H₂ S). "A Mathematical Model of a Catalytic Membrane Reactorfor the Decomposition of NH₃," J. Membrae Science, 78:265-282 (1993).These substances must be removed from the synthetic gas stream.Otherwise, toxic nitrogen oxides (NO_(x)) are formed when the syntheticgas is burned. The temperatures involved with the process aresignificantly greater than can be used with conventional gas separationtechniques. These temperatures also are significantly greater than themelting or combustion point of most organic membrane materials, whichhave an upper useful temperature range of about 150°-200° C.

Three commonly used types of membranes include polymers, ceramics andmetal membranes, such as palladium or palladium-alloy membranes. Each ofthese types of membranes have characteristics that prevent their use forseparating hydrogen from a mixture of gases at high temperatures andpressures. The low thermal and mechanical strength of polymer membranesmakes them unsuitable for reactions involving gases at high pressuresand temperatures. The palladium or palladium-alloy membranes areimpractical on an industrial scale because of their expense, lowhydrogen flux, and because metals deform at high temperatures. As usedherein hydrogen flux or hydrogen permeation rate means the amount ofhydrogen (molar-flow rate) per unit area of membrane. It is important tonote the difference between hydrogen flux and hydrogen permeability.Hydrogen permeability is an intrinsic property of a metal that is usedto determine hydrogen permeation rates at a particular hydrogen partialpressure driving force. Ceramic membranes are able to endure hightemperature and pressure conditions. However, ceramic membranes allowthe mixture of ammonia, hydrogen and nitrogen to flow through themembrane, rather than selectively allowing hydrogen to flow through themembrane. Hence, porous ceramic membranes are insufficiently selectivefor separating hydrogen from other gases at high temperatures andpressures.

Some of the problems associated with previous membranes have beenovercome by combining different types of membranes to increase theoverall efficiency of the combined membrane. For instance, metals, suchas palladium, have been combined with a porous-glass membrane. Morespecifically, a thin palladium film of 20 μm or less has been depositedon the outside surface of a porous-glass tube. This overcomes the lowthermal stability associated with metals while increasing the rate ofhydrogen flux over glass membranes alone. Uemiya et al., "APalladium/Porous-Glass Composite Membrane for Hydrogen Separation,"Chem. Letters, 1687-1690 (1988). However, the glass support does notprovide sufficient stability to be used at high temperatures andpressures. Furthermore, Buxbaum et al. produced a membrane comprising a2 μm palladium film on a niobium disk. However, the Buxbaum et al.procedures are not feasibly applied to extractions of hydrogen attemperatures above about 500° C. At such temperatures,intermetallic-diffusion rates increase, thereby increasing the rate atwhich the niobium diffuses into the palladium metal. Such membraneseventually become impermeable to hydrogen.

Composite palladium-ceramic membranes also have been made. For instance,Uemiya et al. (Uemiya) described the formation of a compositeceramic-palladium metal membrane that was used for the aromatization ofpropane. Uemiya et al., "Aromatization of Propane Assisted by PalladiumMembrane Reactor," Chem. Letters, 1335-1338 (1990). Uemiya formed thecomposite membrane by depositing a palladium metal layer on the outsidesurface of a ceramic tube. Uemiya taught a palladium metal layer havinga thickness of 8.6 μm. Moreover, Uemiya specifically stated that thepromoting effect of the membrane increased with decreasing palladiumthickness. However, Uemiya teaches nothing about (1) selectivelyremoving hydrogen from a mixture of gases at high temperatures andpressures, or (2) selectively separating hydrogen from a mixture ofgases to promote the decomposition of ammonia. The process described byUemiya used a transmembrane pressure of about 1 atmosphere. Palladiumfilm thicknesses of less than about 10 μm have defects when thepalladium film is deposited using an electroless deposition process suchas taught by Uemiya. These defects reduce the effectiveness forselectively removing hydrogen from a mixture of gases at transmembranepressure differences of greater than about 1,000 kPa. Thus, the Uemiyamembrane would not work for removing hydrogen selectively from a mixtureof gases at high temperatures and transmembrane pressures such as wouldbe encountered in the IGCC process or a process whereby hydrogen gas isformed by decomposing ammonia.

Hence, a need exists for a membrane that selectively removes hydrogenfrom a mixture of gases at high temperatures and pressures via asemipermeable membrane.

SUMMARY OF THE INVENTION

The present invention provides a palladium-ceramic membrane that solvesthe problems associated with prior membranes. The palladium-ceramicmembranes of the present invention are primarily useful for selectivelyremoving hydrogen from a mixture of gases, particularly at elevatedtemperatures, such as greater than about 500° C. The composite membranescomprise a porous, tubular asymmetric ceramic support having a pore sizeof greater than about 10 nm. The ceramic support has both an insidesurface and an outside surface. A palladium metal layer is deposited onthe inside surface of the ceramic support by immersing the support in aseries of baths.

The first bath is a sensitizing bath. This bath apparently deposits alayer of colloidal tin ions on the inside surface of the ceramic filter.This allows an activation bath to deposit a uniform layer of finelydispersed palladium crystals on the ceramic surface. Finally, apalladium plating bath is used to deposit a uniform layer of palladiummetal on the activated ceramic surface. The palladium metal layer shouldhave a thickness of greater than about 10 μm, and even more preferablyfrom about 11 μm to about 20 μm. Membranes of the present invention havea hydrogen-to-nitrogen selectivity that is greater than about 380 at atemperature of about 500° C. and a transmembrane pressure of about 1500kPa. Moreover, if the palladium layer has a thickness of about 11 μm,then the hydrogen permeation rate is about 1.25 moles/m².s at atemperature of about 550° C.

The present invention also provides a method for separating hydrogenfrom a mixture of gases. The method comprises first providing a mixtureof gases containing hydrogen. A membrane also is provided. The membranecomprises: (a) a porous tubular ceramic support having a pore size ofgreater than about 10 nm, the ceramic support having an inside surfaceand an outside surface; and (b) a palladium metal layer inside surfaceof the ceramic support wherein the thickness of the metal layer isgreater than about 10 μm. A mixture of gases is flowed past the membraneso that the mixture contacts the inside surface of the ceramic support,thereby selectively removing hydrogen gas from the mixture of gases. Themethod may further include the step of heating the mixture of gases to atemperature of greater than about 400° C. to about 1000° C. if thehigh-temperature sealant can withstand temperatures of about 1,000° C.,more preferably from about 550° C. to about 640° C., before the gasesare flowed past the membrane.

The mixture of gases may include ammonia. If so, then the ammonia isdecomposed to provide nitrogen hydrogen. The hydrogen is selectivelyremoved from the mixture by flowing the mixture past the membrane. Whenammonia is decomposed, the method typically includes the step of placinga catalyst inside the tubular ceramic support and adjacent the insidesurface. The catalyst is placed inside the ceramic filter before thestep of flowing. A presently suitable catalyst for this purpose is anickel catalyst. The mixture of gases may be a mixture of exhaust gasesfrom the IGCC process.

A first object of the present invention is to selectively removehydrogen from a mixture of gases.

A second object of the present invention is to provide aceramic-palladium membrane that can selectively remove hydrogen from amixture of gases at elevated temperatures and pressures that aretypically used in industrial processes such as the IGCC process.

Still another object of the present invention is to provide a method fordecomposing ammonia by catalyzing the decomposition of ammonia tonitrogen and hydrogen, and thereafter promoting the decomposition byselectively removing hydrogen from the mixture of gases.

A first advantage of membranes within the scope of the present inventionis that such membranes can operate at high temperatures and pressuresand still maintain a high hydrogen selectivity.

A second advantage of the present invention is that the membranes can beused in a membrane reactor for the decomposition of ammonia,particularly at high temperatures and pressures such as may beassociated with industrial processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph of a palladium film depositedon a 10 nm pore size asymmetric AI₂ O₃ support.

FIG. 2 is a schematic drawing of a gas-flow system used to evaluate thepermeability and selectivity of membranes made according to the presentinvention.

FIG. 3 is schematic side sectional view of a membrane module used forpermeation trials with membranes according to the present invention.

FIG. 4 is a graph of the hydrogen permeation rate for a compositemembrane made according to the present invention having a 17 μmpalladium layer.

FIG. 5 is a graph of the hydrogen permeation rate for a compositemembrane made according to the present invention having a 11.4 μmpalladium layer.

FIG. 6 is a graph of the hydrogen selectivity at different temperaturesand transmembrane pressures for a composite membrane made according tothe present invention having a 11.4 μm palladium layer thickness.

FIG. 7 is a graph of the normalized hydrogen permeation rates atdifferent transmembrane pressures for composite membranes made accordingto the present invention having palladium-layer thicknesses of 11.4 μm,17 μm and 20 μm.

FIG. 8 is a schematic drawing of flow streams for a membrane reactorsystem.

FIG. 9 is a graph of temperature versus percent-ammonia decompositionfor a membrane reactor as compared to a conventional reactor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention concerns hydrogen-selective ceramic membranes. Aprimary application for such membranes is high temperature hydrogenseparations and the promotion thereby of ammonia decomposition.

The membranes comprise a thin layer of a hydrogen permeable metal, suchas palladium, that is deposited on the inside of a tubular ceramicfilter. The metal layer is supported by the porous ceramic tube. Themetal layers preferably are deposited on the inside of the ceramic tubebecause that is where the smallest pores of the ceramic filter arelocated. Using such membranes provides a hydrogen flux and hydrogenselectivity that is significantly higher than previously reported.

I. MAKING METAL-PLATED POROUS CERAMIC MEMBRANES

Preparing the metal-ceramic membranes within the scope of this inventioncomprises the four following steps: (1) preparing sensitizing,activating and plating solutions; (2) pretreating the membrane; (3)activating the membrane; and (4) plating ceramic membranes using anonelectrolytic process.

The ceramic tubes are first placed in the sensitizing bath. This bathapparently deposits a layer of colloidal tin ions on the inside surfaceof the ceramic filter. This allows an activating bath to deposit auniform layer of finely dispersed palladium crystals on the ceramicmembrane surface. Finally, the plating bath is used to deposit a uniformlayer of palladium metal on the activated ceramic surface. The palladiummetal layer should have a thickness of greater than about 10 μm,preferably from about 11 μm to about 20 μm. Each of these steps isdescribed below in more detail.

A. BATH/SOLUTION PREPARATION

Three baths typically are used for membrane preparation. The first is asensitizing bath, the second is an activating bath, and the third is aplating bath.

1. Sensitizing Bath

Two solutions are used to make the sensitizing bath. The first solutionis an aqueous solution of tin (IV) chloride pentahydrate (SnCl₄.5H₂ O).The second solution is a concentrated inorganic acid solution of tin(II) chloride dihydrate. A suitable inorganic acid is concentratedhydrochloric acid. Both SnCl₄.5H₂ O and SnCl₂.2H₂ O are commerciallyavailable. One skilled in the art will realize that the concentration ofthe tin chloride solutions may differ from the stated values and stillfunction correctly. However, the concentration of the first solution oftin (IV) chloride typically is from about 0.05 M to about 0.15 M, morepreferably about 0.10 M. The second solution is a concentrated solutionof tin (II) chloride that typically has a concentration of about 2.6 M.The first solution generally is allowed to age for a sufficient periodof time to form a colloidal suspension. The aging period may vary, buttypically is for a period of about one week. Thereafter, the first andsecond solutions are combined in deionized water. More specifically, anaqueous sensitizing solution is formed using the aged tin (IV) and tin(II) chloride solutions. A presently suitable sensitizing solutioncontains from about 5 to about 7 volume percent, preferably about 7.5volume percent, of the second solution, and about 4.0 to about 6.0volume percent, preferably about 5.0 volume percent, of the aged firstsolution in deionized water. Example 1 below provides an example of howa sensitizing bath was made according to the present invention.

EXAMPLE 1

20.9 grams of SnCl₄.5H₂ O (MW=350.58, 0.0596 moles) was dissolved in 1liter of deionized water. This solution was allowed to age for one week.After about one week of aging, a colloidal solution was formed. Thissolution can be stored indefinitely before it is used to make thesensitizing bath.

587 grams of SnCl₂.2H₂ O (MW=225.63, 2.60 moles) was dissolved in 780 mlof concentrated hydrochloric acid. The volume of the resulting solutionwas about 1 liter.

The sensitizing bath was prepared approximately one to about two hoursprior to the activation process. To prepare the sensitizing bath, 96.25ml of deionized were added to a bath container, such as a closedcontainer. Thereafter, 8.25 ml of aged SnCl₂.2H₂ O solution were addedto the deionized water, followed by 5.5 ml of SnCl₂.2H₂ O solution. Thefinal volume of the sensitizing bath was approximately 110 ml. The bathtypically is contained in a glass jar or beaker. After the bath isprepared, it is periodically shaken to keep the colloidal suspensionevenly distributed. Although the shelf life of the sensitizing bath maybe greater than about one day, fresh sensitizing baths typically areused on a daily basis.

2. Activation Bath

The activation bath is a dilute acidic solution of palladium chloride(PdCl₂). Palladium chloride is commercially available from a number ofsources. Presently, a particularly suitable palladium chlorideconcentration for this bath has been found to be about 0.5 to about 5.0millimolar, preferably about 1.5 millimolar. Example 2 describes thepreparation of such an activation bath.

EXAMPLE 2

5 ml of concentrated hydrochloric acid were added to about 995 ml ofdeionized water. Thereafter, 0.267 g of palladium chloride (PdCl₂,MW=177.31) was added to the hydrochloric acid solution. The solution wasallowed to sit for several hours until the palladium chloride wassubstantially completely dissolved. The resulting solution can be storedindefinitely.

The solution can be used as is or can be diluted with deionized water ifnecessary. When dilution is desired or required, the solution is dilutedusing deionized water. The shelf life of the activation bath has notbeen determined. Presently, a fresh activation bath preferably is usedon a daily basis.

3. Plating Bath

Activated palladium metal is used to form the membrane-metal composites.One skilled in the art will realize hydrogen-permeable metals other thanpalladium may be used for this invention. Examples of other metals thatmay be used include palladium alloys, such as palladium-gold andplatinum. A presently suitable source of palladium for the plating bathis a tetramine palladium chloride complex [Pd(NH₃)₄ Cl₂ ]. In order todeposit palladium from an aqueous solution, the palladium is preferablystabilized by forming as a palladium complex. One skilled in the artwill realize that the complex is not limited to the tetramine complexdescribed herein. However, the tetramine complex is a presently suitablesource of palladium.

The tetramine complex is formed by adding ammonia to an acidic stocksolution of palladium chloride. A presently suitable tetramine complexmay be prepared by adding 28 weight-percent ammonia to the acidicpalladium chloride stock solution. The concentration of this solutionmay vary and still function for its intended purpose. However, aparticularly suitable palladium (II) chloride concentration has beenfound to be about 0.05 to about 0.06 molar.

The palladium baths are prepared by first adding about 1.75 g of thesodium salt of ethylene diamine tetracidic acid (Na₂ EDTA) to about 25ml of the tetramine palladium (II) chloride solution. This solution isthereafter allowed to sit for a short period of time before actuallyusing the solution to plate a ceramic tube. This period of time is fromabout 0.25 hours to about 3.0 hours, more preferably about 2.0 hours.Typically, immediately prior to a plating trial, about 0.25 ml of a 1.0M hydrazine solution are added to the solution. The hydrazine acts as areducing agent to reduce the palladium (II) to palladium (0). Example 3describes the preparation of such a plating bath.

EXAMPLE 3

An acidic palladium stock solution first was formed by adding 20 ml ofconcentrated hydrochloric acid to approximately 900 ml of deionizedwater. Thereafter, 10 g of palladium chloride (MW=177.31, 0.0564 moles)were added to the acidic solution. The solution was allowed to sit forseveral hours until the palladium chloride was substantially completelydissolved. A tetramine complex solution was prepared by adding 120 ml ofdeionized water to 1 liter of the palladium-chloride stock solution.Thereafter, 715 ml of 28-weight-percent ammonium hydroxide were slowlyadded to the stock solution. This solution was allowed to sit for about3 days.

A palladium bath was prepared by first adding 1.75 g of the sodium saltof ethylene diamine tetracidic acid (Na₂ EDTA) to 25 ml of complexsolution. This solution was thereafter allowed to sit for 2 hours beforeplating. Immediately prior to a plating trial, 0.25 ml of a 1.0 Mhydrazine solution were added to the solution.

TABLE I below shows the composition of such an electroless platingcomposition.

                  TABLE I                                                         ______________________________________                                        Typical Electroless Plating Bath Composition                                  Component              Concentration                                          ______________________________________                                        palladium chloride     5.4 g/L                                                ammonium hydroxide (28 percent)                                                                      390 mL/L                                               disodium EDTA          70 g/L                                                 hydrazine (1 molar solution)                                                                         10 mL/L                                                pH                     11                                                     Temperature            70 to 100° C.                                   Plating surface area   527 cm.sup.2 /L                                        ______________________________________                                    

B. CERAMIC MEMBRANES

Ceramic membranes are used primarily because of their structuralintegrity, their refractory nature, and chemical inertness. As suedherein, "ceramic" refers to filters such as Al₂ O₃ filters. The term"ceramic" does not include glass filters. Moreover, ceramic membranesprovide a suitable porosity for metal/ceramic membrane compositesaccording to the present invention. Ceramic membranes provide bettersupport than do glass membranes. Therefore, ceramic may be used forhigher temperatures and transmembrane pressures required for industrialprocesses.

Suitable ceramic membranes for the present invention may be purchasedfrom the U.S. Filter Corporation of Warrendale, Pa. Particularlysuitable ceramic membranes may be obtained from the U.S. FilterCorporation as MEMBRALOX® T170 ceramic filters. However, one skilled inthe art will realize that other ceramic filters also may be suitable forthis invention. A second example of a filter that may be suitable is amultiple-channel ceramic membrane monolith, which also available fromthe U.S. Filter Corporation. MEMBRALOX® T170 ceramic filters are alumina(Al₂ O₃) support-tube filters. The support tube has an inner surfacethat is covered by a thin multiple-layer microporous membrane. The poresize of the membrane may vary. "Pore size" as referred to herein refersto the pore diameter of the inside layer of the composite ceramicmembrane discussed above, because this is the selective layer. Suitablepore diameters for this invention are from about 10 nm to about 200 nm.Presently preferred membranes have a pore size closer to about 200 nmbecause these ceramic membranes were found to be more durable than the10 nm pore membranes.

MEMBRALOX® T170 ceramic filters are available in lengths of from about25 to about 75 cm. For typical applications according to the presentinvention, ceramic filters having lengths of 5 to about 6 cm were used.Tubes having a length of from about 5 to about 6 cm were convenient forboth plating and permeation trials. However, one skilled in the art willrealize that tubes of various sizes may be used. Tubes having a lengthof from about 5 to about 6 cm were convenient for both plating andpermeation trials. There is nothing critical about the 5 to 6 cm tubelength discussed herein. Prior to plating, the ceramic filters were cutto the desired length using a diamond saw. The filters thereafter weresanded and cleaned.

1. Membrane Pretreatment

In general, membrane pretreatment involves cutting the ceramic tubes,polishing the tubes, and sealing the ends of the tubes. One skilled inthe art will realize that more than one cleaning procedure will work.However, a particularly suitable cleaning procedure involves thefollowing steps: first, the ceramic tubes are rinsed ultrasonicallyusing a sonicator and deionized water. Ultrasonic rinsing is continuedfor approximately five minutes. Thereafter, ultrasonic rinse isperformed using an alkaline solution. After the ceramic tubes are clean,the ends of the tube are sealed using a sealant that can withstandhigh-temperatures, i.e. greater than about 527° C. The ends of theceramic tubes are sealed to prevent the gas from passing through themembrane inlet or outlet. The sealant typically is applied to each endof the ceramic tube, from about 0.25 to about 1.0 cm from each end ofthe inside of the tube. The sealant also is applied around the outerends and the outside of the tube at a distance from about 1.5 to about2.0 cm from each end of the ceramic filter. Although any method ofapplying the sealant will suffice, a suitable method for applyingsealant is by using a fine paint brush.

After the sealant is applied, it is typically cured at room temperaturefor about one hour. Thereafter, the sealant-coated ceramic filters areheated to a temperature of from about 750° C. to about 800° C., morepreferably about 780° C., at a heating rate of about 6° C./minute ramprate. The oven is held at a presently preferred temperature of about780° C. for a period of time of from about 10 to about 20 minutes, morepreferably about 15 minutes. Thereafter, the oven is allowed to cool toa temperature of about 100° C. The sealing procedure is repeated. Thus,a presently suitable sealing procedure involves applying more than onecoat of sealant, and preferably involves applying at least two coats ofsealant. Example 4 provides an example of membrane pretreatment.

EXAMPLE 4

MEMBRALOX® Ceramic tubes were obtained from the U.S. Filter Corporation.The tubes were cut to a length of about 5 to about 6 cm using a diamondsaw. After the T170 filters were cut, the filters then were sanded toobtain an outer diameter that fit into compression fittings. The outsidediameter of the sealed ceramic filters was decreased to 9.7 mm bysanding the outside of the ceramic tube. During sanding, one end of eachceramic tube was secured in a 10 mm to 1/4×" SWAGELOK reducing unionhaving nylon ferrules. A 1/4" outside diameter stainless-steel tube wasattached to the 1/4"-end of the reducing union. The stainless-steel tubethen was put in a drill chuck. The drill was set to a low speed and theoutside of the tube is gently sanded until the outside diameter of theends were about 9.7 mm. This sanding procedure was performed on eachend.

The tubes were then placed in a sonicator containing deionized water.The tubes were left in the sonicator for a period of about 15 minutes.An alkaline cleaning solution was prepared by dissolving approximately0.25 g ALCONOX in about 250 ml of deionized water. ALCONOX is adetergent that can be obtained from a number of commercial vendors. Thedeionized water was at a temperature of about 50° C. before the ALCONOXwas added. Thereafter, 10 ml of 28-weight-percent ammonia were added tothe solution. 250 ml of cold deionized water were added to complete thesolution.

A sufficient amount of this alkaline solution was added to substantiallyfill the sonicator, and the ceramic tubes then were placed in thesolution. The sonicator was continued for a period of five minutes. Theceramic filters were thereafter rinsed with cold deionized water for oneminute. Thereafter, the ceramic filters were immersed in acetic acid (25wt. %) for five minutes. The ceramic filters were again ultrasonicallyrinsed in cold deionized water for three minutes, followed by anultrasonic rinse in 60° C. deionized water for one minute. Finally, theceramic filters were ultrasonically rinsed in isopropyl alcohol for fiveminutes.

After the ceramic tubes were cleaned according to the process describedabove, they are then sealed a high-temperature (above about 500° C.)sealant. "Sealed" as used herein refers to coating each end of theceramic tube with a sealant that can withstand temperatures of greaterthan about 500° C. This prevents gases from leaking through the ceramicmembrane when the ends of the ceramic tube are sealed with compressionfittings and the ceramic tube is charged with a mixture of gases athigher temperatures. A presently suitable sealant is an Aremco 617sealant. The sealant is applied to each end of the ceramic tube from adistance of about 0.25 to about 1.0 cm from each end of the inside ofthe tube. The sealant also was to the outside of the tube at a distancefrom about 1.5 to about 2.0 cm from each end of the ceramic filter.Although any method of applying the sealant will work, a suitable methodfor applying sealant is by painting the sealant on the ceramic tubeusing a fine paint brush. The sealant is cured at room temperature forabout one hour. Thereafter, the sealant-coated ceramic filters wereheated to about 780° C. at a heating rate of about 6° C./minute. Theoven was held at a temperature of about 780° C. for a period of time ofabout 15 minutes. The oven was allowed to cool to a temperature of about100° C. The sealing procedure was then repeated.

2. Membrane Activation

The inside surface of the ceramic membrane must be uniformly seeded witha metal, such as palladium crystals. The selective membrane of theceramic filter is on the inside of the tube. Plating the ceramic tuberequires using both the sensitizing and activation baths describedabove. The sensitizing bath deposits a colloidal layer of tin ions onthe inside surface of the ceramic tube. The tin layer provides afoundation for depositing palladium at discrete locations on the tinsurface using an activation bath which presently is a dilute solution ofPdCl₂. Finally, a plating bath is to deposit a uniform layer ofpalladium metal on the activated ceramic surface.

It is important to prevent the activation of the outside surface of themembrane. Hence, the outside surface of the ceramic filters typicallyare covered prior to seeding with palladium crystals. Presently, aparticularly suitable coating method comprises wrapping the outsidesurface of the tube with TEFLON tape.

EXAMPLE 5

To activate the membrane, the ceramic filter first was soaked in asufficient amount of sensitizing bath to substantially cover the tubesimmersed therein. The tubes were left in the sensitizing bath for aperiod of about five minutes. Thereafter, the ceramic filter first wasrinsed with deionized water, and then immersed in an activating bath forabout five minutes. Again, the ceramic filter was rinsed with deionizedwater after being removed from the activation bath. This procedure wasrepeated seven times, although the actual number of platings may varyfrom trial to trial. The purpose of repeated platings is to achieve auniform distribution of the palladium crystals on the inside surface ofthe ceramic membrane. Presently, a suitable number of coatings is aboutseven times. The activated surface was light brown in appearance.Following activation, the TEFLON tape was removed and the membrane wasrinsed in deionized water.

3. Membrane Plating

TEFLON tape again is wrapped around the outside of the tube to protectthe sealant from the plating bath. Hydrazine is added to the platingbath immediately prior to immersing the membrane therein. Although anysuitable container will suffice for the plating bath, a particularsuitable plating bath container was a 30 ml glass vile having a screw oncap. The membrane is added to the 30 ml glass vile, and the was looselyscrewed back on the container. The container thereafter is heated to atemperature of from about 70° to about 80° C. Presently, a particularlysuitable method for heating the closed container comprises placing theclosed container in a water bath heated to about 70° to about 80° C.Heating the solution during the plating procedure to a temperature offrom about 70° C. to about 80° C. appears to provide a more uniformdeposit of palladium. The vile is gently shaken periodically. Themembrane is allowed to remain in the heated water bath for a period oftime of about one hour.

After the first plating procedure, the membrane is rinsed off usingdeionized water, and fresh TEFLON tape is applied. The ceramic filterthen is placed in a fresh plating bath for another hour of plating. Thisprocess is repeated until a desired plating film thickness is obtained.An approximate plating deposition rate is about 2.5 μm per hour when 6cm samples are plated. After plating, the membrane is again rinsed withdeionized water and then dried at a temperature of about 110° C. Thepalladium film thickness can be estimated by measuring the weightdifference between the initial and final membranes.

The thickness of the deposited palladium layer may be greater than about10 μm to about 20 μm.

FIG. 1 is a scanning electron micrograph of asymmetrical Al₂ O₃ membranehaving a 10 nm pore top layer. FIG. 1 shows that a palladium layeractually was deposited on the inner surface of a ceramic filter usingthe process of the present invention. The top horizontal band or sectionshown in FIG. 1 is the porous supportive ceramic membrane. The bottomsection represents the palladium layer. The thickness of the palladiumlayer deposited on the ceramic membrane shown in FIG. 1 was about 1.5μm.

Palladium plated ceramic membranes made according to the methoddescribed above have been tested determine whether they delaminate.Palladium plated ceramic membranes have been heated to about 600° C. andcooled with no observable delamination of the metal films.

II. MEMBRANE CHARACTERIZATION

Membranes were tested to determine their hydrogen permeability andselectivity. These tests were conducted using hydrogen, nitrogen andhelium gases.

The apparatus used to test the permeability and selectivity of membranesis shown in FIGS. 2 and 3. FIG. 2 is a schematic representation of aflow system used for the permeation trials. FIG. 2 shows that nitrogenhydrogen and helium gases were connected to a membrane module 10. Themembrane module 10 is shown in more detail schematically in FIG. 3.Sources of hydrogen, helium and nitrogen gases were connected to themembrane module using feed lines and mass flow controllers. Although anysuitable mass flow controller may be used for this invention,particularly suitable mass flow controllers are Brooks 5850E mass flowcontrollers. These controllers were used to regulate the gas flow, aswell as determining the gas compositions of the gas introduced to themembrane module 10. Helium was used as both a sweep gas and as amembrane feed gas, whereas nitrogen and hydrogen were used solely asmembrane feed gases. The feed gases were fed through the membrane module10 shown in FIG. 3. After the gas passed through the membrane module 10,it was then fed to a 10-port sampler. From the sampler, the exhaustedfeed gas was analyzed by a Hewlett Packard 5890 Series II chromatographhaving TCD detectors.

The permeability of membranes made according to this invention weretested using a membrane module as shown in FIG. 3. Module 10 comprises atube inlet 12 that is connected to the feed gases using any suitableconnector. Module 10 also includes a shell inlet 14 for the sweep gas,which typically is helium. As used "sweep gas" refers to a gas thatpasses through shell inlet 14 and passes by the ceramic membranesadjacent to the outside surface of the membrane. The inlet gas is fedthrough the interior of the ceramic membrane adjacent to the inside,palladium coated surface of the membrane. Controlling the flow throughthe sweep inlet 14 is a valve 16. Tube inlet 12 is connected to a tube18 using compression fittings. Although one skilled in the art willrealize that any suitable compression fitting may be used for thisinvention, particularly suitable compression fittings are SWAGLOK®compression fittings. The SWAGLOK® fittings include Grafoil or graphiteseals.

Tube 18 typically is an alumina tube or a metal tube having an outerdiameter of about 2.5 cm. Located inside tube 18 is a first thermocouple22. This thermocouple 22 is used to monitor the temperature of the inletgas. A presently suitable thermocouple is a Type K thermocouple, whichis available from Omega. Also located inside the tube 18 is a secondalumina tube 24 that is connected to the inlet 12. Tube 24 presently isa 0.64 cm outer-diameter nonporous alumina tube. Tube 24 passes throughthe center of tube 18 and through cap 26. Furthermore, tube 24 leads toa first end of a membrane 28 made according to the present invention.The membrane 28 is connected to alumina tube 24 using metal compressionfittings 30. These compression fittings also include Grafoil or graphiteseals (not shown). The alumina tube 24 also is connected to a second endof the membrane 28, again using compression fittings and Grafoil orgraphite seals. Tube 24 continues through the center of tube 18.

A second thermocouple 32, preferably a Type K thermocouple, ispositioned to monitor the temperature of the outlet gas as it passesfrom the membrane 28. The temperature of the gas at the inlet was higherthan at the outlet, presumably because of a non-uniform temperaturedistribution within the furnace 40. The temperature difference measuredbetween the inlet and the outlet was about 20° C. Hence, membranetemperature as referred to herein means the average of the inlet andoutlet temperatures. Tube 24 thereafter passes through a cap 42 andthrough tube outlet 34. Only the gas that passes through membrane 28passes through outlet 34. The sweep gas is exhausted out shell outlet36. Tube 24 also is connected to a temperature controller 38. Thistemperature controller 38 was used to regulate the temperature of themembrane 28. A particularly suitable temperature controller is an OmegaCN9000 controller. The membrane module 10 is located inside a hightemperature furnace 40. This furnace 40 is used to heat the membrane tothe desired working temperature.

1. Permeability Tests

Permeability tests were performed on various composite membranes. Themembranes were tested at numerous temperatures ranging from about 450°C. to about 640° C. The upper temperature limit was determined primarilyby the operating temperatures of the Aremco 617 end seals. The gas feedpressures also were varied. The feed pressures ranged from about 156 kPato about 2445 kPa, whereas permeate pressures ranged from about 101 kPato about 140 kPa. Transmembrane pressure differences ranged from about40 kPa to about 2330 kPa.

Gas permeabilities were determined by flowing a substantially pure gasthrough the membrane at various pressures. When hydrogen and helium weretested, or when mixtures of hydrogen, nitrogen and helium were tested, asweep gas was not employed. However, a sweep gas was used to test thepermeability of nitrogen.

Membranes having palladium layers of from about 10 μm to about 20 μmhave been made and tested. Membranes having palladium films of fromabout 11.4 μm to about 20 μm were used for the high-temperature tests,that is temperatures greater than about 427° C. Prior to subjecting themembranes to high-temperature tests, the membranes first were subjectedto ambient temperature tests. These tests were conducted by pressurizingmembranes with nitrogen up to a pressure of about 240 kPa. Thepressurized tubes then were immersed in water to determine if there wereany leaks in the membranes tested. These tests indicated that membraneshaving a palladium layer less than about 10 μm thick leaked nitrogen.Hence, membranes having a palladium-layer thickness of less than about10 μm would have a lower hydrogen selectivity than membranes having agreater thickness. For this reason membranes having palladium layerswith a thickness less than about 10 μm were not tested at elevatedtemperatures. Moreover, membranes having a palladium layer of greaterthan about 10 μm are preferred membranes for high-temperature,high-pressure gas separations due to their higher hydrogen selectivity.

To permeate through the composite membranes of the present invention,the gas must permeate through both the palladium metal film and theceramic membrane support. Hence, hydrogen permeabilities for 4 nmMEMBRALOX® ceramic supports were estimated using the permeability datapresented by Wu et al. Wu et al., "High-Temperature Separation of BinaryGas Mixtures Using Microporous Ceramic Membranes," J. Membrane Sci.,77:85-98 (1993). It is important to note that Wu et al. used only a 4 nmceramic membrane, as opposed to a composite metal-ceramic membrane. Withsuch a membrane the hydrogen permeation rate is on the order of about 23to 44 times the permeation rate for composite membranes made accordingto the present invention. At low transmembrane pressure differences andhigh temperatures, gas transport through these ceramic membranes occursprimarily by Knudsen flow. The hydrogen/nitrogen selectivity underKnudsen flow conditions is only 3.74. As the transmembrane pressuredifference increases, viscous flow also becomes which results in adecrease in the already-low hydrogen selectivity. Therefore, thehydrogen selectivity of the ceramic membrane is much too low to make itfeasible to use in a membrane reactor for ammonia decomposition and theseparation of hydrogen from gas mixtures at high temperatures andtransmembrane pressure differences. Moreover, because the permeationrates of Wu et al. are so high, it was assumed that the mass transferresistance of the ceramic support is minimal, and that the hydrogenpermeability rates measured using the composite membranes of the presentinvention reflect the permeability of the palladium film deposited onthe ceramic support.

Either pure hydrogen, nitrogen or helium was fed to the membrane.Alternatively, mixtures of hydrogen, nitrogen and helium were usedhaving various mole-percent compositions. It was important to determinethe composition of the outlet gas streams. To determine the compositionof such streams, the outlet gas streams were coupled to bubbleflowmeters and a gas chromatograph as discussed above. In general, largeamounts of hydrogen gas were permeated to the sweep side relative tonitrogen and helium gas.

The results of hydrogen permeation rates with three composite membranesare shown below in TABLE II.

                  TABLE II                                                        ______________________________________                                        Hydrogen Permeabilities at Specific Temperatures for                          Composite Palladium-Ceramic Membranes                                         Membrane Number and                                                                         Temp-   P.sub.H                                                 Description   erature (moles · m/m.sup.2 · s ·                           Pa.sup.n)       n                                       ______________________________________                                        1. 20 μm palladium                                                                       550     1.43 · 10.sup.-8                                                                     0.526                                   film on ceramic                                                               membrane with 10 nm                                                           pore layer                                                                    2. 17 μm palladium                                                                       450     2.34 · 10.sup.-9                                                                     0.622                                   film on ceramic                                                                             500     4.04 · 10.sup.-9                                                                     0.595                                   membrane with 200                                                                           550     6.82 · 10.sup.-9                                                                     0.568                                   nm/// pore layer                                                                            600     9.96 · 10.sup.-9                                                                     0.552                                   3. 11.4 μm 550     3.23 · 10.sup.-9                                                                     0.602                                   palladium film on                                                                           600     5.84 · 10.sup.-9                                                                     0.566                                   ceramic membrane                                                              with 200 nm pore                                                              layer                                                                         ______________________________________                                         Note:                                                                         Hydrogen permeabilities calculated from nonlinear regression of Equation      using permeating data collected at each temperature.                     

Equation 1 below describes the hydrogen flux (hydrogen permeation ratethrough a palladium film):

Equation 1

    J=P.sub.H /t.sub.m (P.sub.Ht.sup.n -P.sub.He.sup.n)

J is the hydrogen permeation rate as expressed in moles/m².s. P_(H) isthe hydrogen permeability at a given in units of moles.m/m².s.Pa^(n).t_(m) is the palladium film thickness in meters. P_(Ht) is the partialpressure of hydrogen on the tube side, whereas P_(Hs) is the hydrogenpartial pressure on the permeate side. Finally, n is the pressuredependence term. A value of 0.5 occurs when permeation through the metalfilm follows Sievert's Law.

The hydrogen permeation rates were dependent on the hydrogen pressure tothe 0.526 to 0.622 power, very close to the 0.5 power dependenceexpected from Sievert's Law. The variation of n with temperature andpalladium film thickness is due to several factors. Permeation ofhydrogen through palladium metal involves several steps (Shu et al.,1991):

1) reversible dissociative chemisorption of molecular hydrogen on themembrane surface;

2) reversible dissolution of surface atomic hydrogen in the bulk layersof the metal;

3) diffusion of atomic hydrogen through the bulk metal.

Steps 1 and 2 take place on both surfaces of the metal. Sievert's Law isonly applicable when diffusion through the bulk metal controls thepermeation rate and hydrogen atoms form an ideal solution in thepalladium metal. Deviation from this behavior increases the value of n.In practice, n depends on temperature since it is influenced by therelative rates of the surfaces processes and diffusion through the bulkmetal, which all depend on temperature. The palladium film thicknessalso impacts n since the relative rates of surfaces processes and bulkdiffusion also depend on the palladium film thickness. Surface processesbecome more important when the palladium film thickness is reduced sincethe diffusion length decreases as the film thickness decreases.

FIG. 4 is a graph of the hydrogen-permeation rate (mole/m² /s) as afunction of temperature between the temperatures of 450° C. and 600° C.More specifically, FIG. 4 shows the permeation rate of hydrogen throughmembrane 2 at 450° C., 500° C., 550° C. and 600° C. This membrane has apalladium layer thickness of about 17 μm. The membrane was operated forover 200 hours at temperatures over about 450° C. without failure. Themembrane was first tested at a temperature of about 450° C., and thentested sequentially at the higher temperatures. Hydrogen permeabilitieswere determined at each of these temperatures using the apparatus shownin FIGS. 2 and 3. As can be seen from the data shown in FIG. 4, thepermeabilities of the membranes increased with increasing temperatures.When the membrane was cooled back down to 450° C., the hydrogenpermeability had increased relative to the permeability initiallymeasured at 450° C. More specifically, the hydrogen permeabilityincreased from about 20 to about 40 percent over the initiallydetermined permeation rate, depending upon the temperature at which thepermeability rate was measured.

Without limiting the invention to one theory of operation, one possibleexplanation for the increased permeation rates was that impuritiesinitially present in the membrane were burned off during the firsthigh-temperature tests. A second possible explanation is that theheating process may have had an annealing effect on the palladiumsurface. For whatever reason, it appears that the permeation rates ofthe membranes may be improved by including a high-temperature heattreatment step. High-temperature as used herein means a temperature ofat least about 400° C., and preferably greater than about 500° C. toabout 640° C.

FIG. 5 shows the hydrogen permeability results for a membrane having apalladium thickness of about 11.4 μm. FIG. 5 shows the data for hydrogenpermeability for this membrane at two temperatures, namely 550° C. and600° C. The hydrogen permeability increases as the temperatureincreases. Furthermore, this FIG. 5 shows that as the hydrogen partialpressure increases across the ceramic membrane, the hydrogen permeationrate also increases.

FIG. 6 shows the hydrogen selectivity data for compositepalladium-ceramic membrane with a 11.4 μm palladium film. "Selectivity"as used herein is defined as the ratio of the hydrogen permeation rateto either the nitrogen or helium permeation rates at the sametransmembrane pressure difference. The hydrogen selectivity formembranes decreases with increasing transmembrane pressure differences.This apparently is because the permeation rate of nitrogen and heliumthrough membrane seals or palladium film defects is proportional to ahigher power of pressure than is the hydrogen permeation rate throughthe palladium film. Defects in the palladium film layer increase withdecreasing thickness which is another reason why palladium layersgreater than about 10 μm are preferred. FIG. 6 shows the hydrogenselectivity of the 11.4 μm palladium layer membrane relative to nitrogenand helium at 550° C. and 600° C. and at different transmembranepressure differences (P_(t) -P_(s)). P_(t) is the total pressure on themembrane side, and P_(s) is the total pressure on the sweep side. Thehydrogen selectivity for this membrane relative to nitrogen ranges fromabout 1200 to about 380 over the transmembrane pressure differencestested. It can be seen from this FIG. 6 that as the pressure differenceincreases the selectivity decreases. It also can be seen from FIG. 6that the membranes are more selective for hydrogen than both helium andnitrogen.

FIG. 7 is a comparison of normalized permeation rates at 550° C. forcomposite palladium-ceramic membranes made according to the presentinvention. "Normalized" as used herein means that the hydrogenpermeation rates were divided by the palladium layer thickness. FIG. 7shows the normalized permeation rates at 550° C. as a function oftransmembrane pressures. This FIG.7 shows that the normalized permeationrates for the 11.4 μm palladium film were significantly lower than therates for membranes having thicker layers of palladium.

The lower normalized permeation rates are believed to be due to theeffect of surface processes on the hydrogen permeation rates for the11.4 μm palladium film. Hydrogen permeation rates should be inverselyproportional to the palladium film thickness if diffusion through thebulk palladium metal controls the permeation rate. The normalizedpermeation rates would then be equal for all palladium film thicknesses.However, surfaces processes start to influence the hydrogen permeationrate when the film thickness is reduced below a certain value. Thismeans that hydrogen permeation rates should approach a limiting valuewhen the palladium film thickness is reduced below a certain point. Thelower normalized hydrogen permeation rates for the 11.4 μm palladiumfilm may indicate that surfaces processes are starting to influence thehydrogen permeation rate. Therefore, normalized hydrogen permeationrates are lower for the 11.4 μm membrane but hydrogen permeation ratesare still higher than for the 17 and 20 μm films. Furthermore,decreasing the palladium film thickness below 11.4 μm should notsignificantly increase hydrogen permeation rates since surfacesprocesses become increasingly important as the film thickness islowered. Thus, it is preferable to use an 11.4 μm palladium film ratherthan a thicker film since higher hydrogen permeation rates are obtainedand the hydrogen selectivity is just as good as with the thickerpalladium films.

TABLE III compares composite membranes of the present invention withmembranes currently available. The membranes tested included: (1) Uemiyaet al.'s composite palladium-porous glass membrane having a 13 μmpalladium film layer ("A Palladium/Porous-Glass Composite Membrane forHydrogen Separation," Chem. Lett., 1687-1690 (1988)]; (2) Edlund's 25 μmpalladium film on vanadium film ["A Membrane Process for Hot-Gas andDecomposition of H₂ S to Elemental Sulfur," Phase I Final Report to theU.S. Department of Energy, (1992)]; (3) Buxbaum's 2 μm palladium film on0.2 cm thick niobium disk ["Hydrogen Extraction Via Non-PorousCoated-Metal Membranes," Preprint for paper presented at AIChE AnnualMeeting (1992)]; (4) Tsapatis et al.'s SiO₂ deposited on Vycor glassmembrane ["Synthesis of Hydrogen Permselective SiO2, T₁ O₂, Al₂ O₃ andB₂ O₃ Membranes from the Chloride Precursors, " Ind Eng. Chem Res.30:2152-2159 (1991)]; and (5) Wu et al.'s asymmetric ceramic membranewith 4 nm pore top layer ["High-Temperature separation of Binary GasMixtures Using Microporous Ceramic Membranes," J. Membrane Sci.,77:85-98 (1993)].

TABLE III shows that the hydrogen permeation rates for the membranesaccording to the present invention are superior to the permeation ratesobtained by other membranes, other than the ceramic membrane made by Wuet al. However, the Wu et al. membrane is a non-selective membrane, andhence is not capable of separating hydrogen from a mixture of gases asare the membranes of the present invention. More specifically, thehydrogen permeation rates for the composite membranes of the presentinvention was found on the average to be about 0.71 moles/m².s at atransmembrane pressure difference of about 690 kPa. The next-bestpermeation rate was obtained by Uemiya et al. at 0.59 moles/m².s. Thispermeation rate is similar to the permeation rate obtained with thecomposite ceramic-palladium membranes of the present invention iftemperature differences are considered. However, the Uemiya membrane isa glass composite. Glass-composite membranes will not withstand the hightemperatures and high pressures used for processes such as the IGCCprocess. The hydrogen permeation rate for Edlund's membrane was about0.30 moles/m².s. Again, this value is significantly lower than thehydrogen permeation rate obtained by the composite membrane madeaccording to the invention, even though the temperature of the Edlundtest was about 150° C. higher. Finally, the hydrogen permeation ratesfor the Buxbaum and Tsepatis et al. membranes are from about seven-foldslower (for the Buxbaum membrane, 0.09 moles/m².s) to about 47 timesslower (for the Tsepatis et al. membrane, 0.015 moles/m².s) than withthe membranes of the present invention.

                  TABLE III                                                       ______________________________________                                        Comparison of Hydrogen Permeation Rates                                       for Inorganic Membranes                                                                              Hydrogen                                                                      Permeation                                                            Temp-   Rate                                                   Membrane Description                                                                         erature (moles/m.sup.2 · s)                                                              References                                 ______________________________________                                        composite palladium-                                                                         823     0.71        this work                                  ceramic membrane                                                              (11.4 μm palladium                                                         film)                                                                         composite palladium-                                                                         773     0.56.sup.b  Uemiya et                                  porous glass membrane              al. (1988)                                 (13 μm palladium                                                           film)                                                                         composite metal                                                                              973     0.30.sup.c  Edlund                                     membrane (25 μm                 (1992)                                     palladium film on 30                                                          μm vanadium foil with                                                      1 μm intermetallic                                                         diffusion barrier                                                             between palladium and                                                         vanadium)                                                                     composite metal                                                                              698     0.09.sup.b  Buxbaum                                    membrane (2 μm                  (1992)                                     palladium film on 0.2                                                         cm thick niobium                                                              disk)                                                                         metal oxide membrane                                                                         723     0.015.sup.b Tsapatis et                                (SiO.sub.2 deposited in            al. (1991)                                 pores of 4 nm Vycor                                                           glass membrane)                                                               ceramic membrane                                                                             811     23.sup.b    Wu et al.                                  (asymmetric membrane               (1993)                                     with 4 nm pore top                                                            layer)                                                                        ______________________________________                                         Footnotes                                                                     .sup.a Permeation rates are based on a feed pressure of pure hydrogen         equal to 790610 Pa and a permeate pressure of 101325 Pa.                      .sup.b Permeation rates estimated from hydrogen permeability data reporte     in cited references.                                                          .sup.c Permeation rate taken directly from cited reference.              

III. MEMBRANE REACTOR

Experiments have been conducted to verify that the membranes madeaccording to the present invention can be used to increase thedecomposition of ammonia relative to conventional techniques. A nickelcatalyst on supported alumina was used to decompose ammonia. The reactorused to test the membranes consisted of a shell and tube configurationas shown in FIG. 8. FIG. 8 shows that the feed gas could include amixture of hydrogen, nitrogen, carbon monoxide, water, hydrogen sulfideand ammonia. Ammonia decomposition experiments used only ammonia,nitrogen, hydrogen and helium as the inlet gas. Either hydrogen sulfideor ammonia can be decomposed using the reactor by choosing anappropriate catalyst. There is some concern that hydrogen sulfide mightpoison a palladium catalyst. However, Goltsov et al.'s U.S. Pat. No.3,881,891 states that adding water vapor to a mixture of gases preventspalladium or palladium-alloy poisoning. The IGCC process provides amixture of gases that includes both hydrogen sulfide and water vapor.The water vapor should prevent palladium poisoning by hydrogen sulfide,thus allowing decomposition of both ammonia and hydrogen sulfide using amembrane reactor within the scope of the present invention. FIG. 8 showsthat hydrogen gas resulting from the decomposition of ammonia permeatesthrough a semi-permeable membrane taught by the present invention.

                  TABLE IV                                                        ______________________________________                                        EXPERIMENTAL CONDITIONS FOR MEMBRANE                                          REACTOR EXPERIMENTS                                                                             Membrane                                                                              Conventional                                                          Reactor Reactor                                             ______________________________________                                        Feed composition (mol %)                                                      NH.sub.3            0.34      0.34                                            N.sub.2             47.6      47.6                                            H.sub.2             20.1      20.1                                            He                  32.0      32.0                                            Feed flow rate (sccm)                                                                             422       422                                             Feed pressure (psig)                                                                              220       220                                             Feed temperature (°C.)                                                                     450-600   450-600                                         Shell-side pressure (psig)                                                                        1.25      --                                              Shell-side inlet flow rate (sccm)                                                                 0         --                                              Reactor tube diameter, inside (cm)                                                                0.7       0.6                                             Reactor length (cm) 5.5       7.5                                             Catalyst weight (g) 1.23      1.23                                            Membrane material   Pd on     --                                                                  alumina                                                                       ultrafilter                                               Membrane thickness (μm)                                                                        11.4      --                                              ______________________________________                                    

The results of ammonia decomposition experiments are shown in FIG. 9.FIG. 9 is a graph showing the percent decomposition of ammonia versustemperature that was achieved with a membrane reactor according to thepresent invention versus a conventional reactor. The conventionalreactor comprised a nonporous alumina tube having dimensions similar tothe membrane reactor. The conventional reactor was packed with asupported nickel catalyst. Equilibrium conversion as referred to in FIG.9 is the maximum ammonia conversion that can be achieved in aconventional reactor of infinite length. FIG. 9 clearly shows that themembrane reactor had significantly increased ammonia decompositionrelative to the conventional reactor. The decomposition of using themembrane reactor was low below a temperature of about 500° C. but wassignificantly increased above a temperature of about 600° C. Morespecifically, at a temperature of about 600° C. the percentdecomposition of ammonia was greater than about 94% using the membranereactor. The results presented in FIG. 9 clearly demonstrate that amembrane reactor is more effective than a conventional reactor, andunder conditions similar to that used in an IGCC can achieve almostcomplete removal of ammonia.

The present invention has been described with reference to preferredembodiments. Other embodiments of the invention will be apparent tothose of skill in the art from the consideration of this specificationor practice of the invention disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with thetrue scope and spirit of the invention being indicated by the followingclaims.

We claim:
 1. A method for removing ammonia from a reaction mixture,comprising the steps of:providing a mixture of gases that includesammonia; providing a composite ceramic-metal membrane that isimpermeable to ammonia and which has a hydrogen-to nitrogen selectivitygreater than achieved by Knudsen diffusion, the membrane comprising (a)a porous tubular ceramic support having a pore size of greater thanabout 10 nm, the ceramic support having an inside surface and an outsidesurface, and (b) a palladium metal layer deposited directly on theinside surface of the ceramic support, the metal layer being uniform andhaving a thickness of from about 10 μm to about 20 μm; placing acatalyst inside the tubular ceramic support and adjacent the insidesurface, the catalyst catalyzing the decomposition of ammonia tohydrogen and nitrogen; flowing the mixture of gases through the tubularceramic support so that the mixture contacts the catalyst, therebydecomposing ammonia to hydrogen gas and nitrogen gas; and separatingsubstantially pure hydrogen gas from the nitrogen gas using theceramic-metal membrane.
 2. The method according to claim 1 wherein themembrane has a hydrogen permeability of greater than about 1.0moles/m².s at a temperature of greater than about 500° C. and atransmembrane pressure difference of about 1,500 kPa.
 3. The methodaccording to claim 1 wherein the hydrogen-to-nitrogen selectivity isgreater than about 600 at a temperature of greater than about 500° C.and a transmembrane pressure of about 700 kPa.
 4. The method accordingto claim 1 wherein the mixture of gases is a mixture of exhaust gasesfrom the IGCC process.